Chromatography, e.g., gas chromatography, gas-liquid chromatography, and/or liquid chromatography, can include separating and/or analyzing the components of a mixture. For example, chromatography can include separating one or more analytes in a mixture, and determining the concentration of the analyte(s), e.g., the relative amount of the analyte(s), in the mixture. An analyte can be, for example, an element and/or compound separated from a mixture for measurement and/or analysis, such as a gas, a liquid, or a combination thereof.
A thermal conductivity detector is one type of device than can be used in chromatography to analyze the components of a mixture. Thermal conductivity detectors can produce a signal that is proportional to the concentration and thermal conductivity of the analyte. For example, thermal conductivity detectors can give equal responses from a high concentration analyte with a low thermal conductivity and a low concentration analyte with a high thermal conductivity. With calibration, thermal conductivity detectors can be used in gas chromatography and/or gas-liquid chromatography to determine the concentration of one or more analytes separated from a mixture.
A thermal conductivity detector can provide high sensitivity for large analyte concentrations. That is, thermal conductivity detectors can distinguish small differences in concentration among highly concentrated analytes. Additionally, a thermal conductivity detector can be a universal detection mechanism. For example, thermal conductivity detectors can determine the concentration of any type of analyte, so long as the analyte's thermal conductivity is different from that of the chromatographic carrier fluid (typically He, N2, or H2).
Previous thermal conductivity detectors can determine the concentration of an analyte by performing a differential measurement using a reference fluid, e.g., a reference gas and/or liquid that contains just a chromatographic carrier fluid and not any analyte. That is, previous thermal conductivity detectors can be referenced and/or differential thermal conductivity detectors. For example, a previous thermal conductivity detector can determine, e.g., measure, the difference in thermal conductivity between a reference fluid and a fluid under analysis that may contain an analyte. The concentration of the analyte can then be determined using the measured thermal conductivity difference. This difference can be more accurate in that it can account for drifts in sensor response caused by internal and/or external effects, e.g., power supply and/or temperature changes.
However, the reference fluid used to perform the differential measurement in the previous thermal conductivity detector can change the composition of the fluid under analysis, which can pose difficulties for any subsequent detectors. For example, a previous thermal conductivity detector may dilute the analyte, e.g., with the reference fluid, by combining both fluids during the course of measurement. Hence, subsequent measurements and/or analysis performed on the fluid by additional detectors may be less reliable and/or accurate, due to the change in composition, e.g., dilution of the fluid under analysis.
Accordingly, additional detectors may be unsuitable to measure and/or analyze the analyte after a previous thermal conductivity detector performs a differential measurement using a reference fluid. This may occur as a result of the subsequent detector being influenced significantly by the flow perturbations caused by the reference fluid existing in the previous thermal conductivity detector. In one case, these perturbations could manifest themselves as a signal oscillation in a subsequent detector that is proportional to the subsequent detector's signal. These and similar incompatibilities can prevent additional detectors from being successfully used in series with previous thermal conductivity detectors that perform a differential measurement using a reference fluid, because the reference fluid may change the fluid's composition and/or flow characteristics.
Additionally, other embodiments of previous thermal conductivity detectors that perform differential measurements using a reference fluid can avoid changing the composition and/or flow characteristics of the fluid under analysis by adding complexity, and/or increasing the number of components of the detector itself, because additional components may be needed to accommodate the reference fluid. The additional hardware required in such a differential measurement can be undesirable from a cost and/or portability standpoint.
Further, previous thermal conductivity detectors that perform a differential measurement using a reference gas may have a low data collection rate, e.g., less than 50 Hertz, because such previous thermal conductivity detectors may have a relatively large size and/or large thermal mass. Such low data collection rates may not be suitable for some high speed and/or high throughput applications.